This invention relates to a class of lead-free or low lead perovskite structure oxide compounds useful as electromechanical actuators. It also relates to composite actuators utilizing these compounds in the form of oriented polycrystals or crystallites. The invention relates to compositions exhibiting improved strain coefficients, strain actuation and other properties relating to piezoelectric or electrostrictive performance. The invention also relates to a method of making an electromechanical device, e.g., a piezoelectric or electrostrictive device, which incorporates oriented crystallites.
Piezoelectric materials respond to the application of stress by generating electrical polarization in the crystal, and generate a strain upon application of electric field. The magnitude of the piezoelectric effect or electromechanical effects, in general, is measured in part by the piezoelectric coefficient dab which is a measure of the strain in the material due to the application of a unit electric field and the electromechanical coupling constant kab, the square of which is a measure of the efficiency of electromechanical energy conversion. The subscript a denotes the direction in which the field is applied and the subscript b denotes the direction of the measured actuation. The widely used PbxZr1-xTiO3 (PZT) family of piezoelectric actuators typically show d33=200–700 pC/N, maximum strains before dielectric breakdown of 0.3% and maximum electromechanical coupling coefficients of k33˜80%.
Single crystal ferroelectric perovskite compounds such as PbxMg1-xNbyTi1-yO3 (PMN-PT) and PbxZn1-xNbyTi1-yO3 (PZN-PT) have recently been shown to have unusually good performance as piezoelectric actuators, with strain coefficients d33 of up to 2500 pC/N, maximum strains of 1.5%, and electrochemical coupling coefficients of k=94%. See Kawata et al., Jap. J. Appl. Phys. 21:1298 (1982). The piezoelectric properties are superior to those of PZT and appear to be much easier to grow as single crystals than many other perovskites, including PZT. At room temperature, these single crystal actuators form the perovskite structure in a phase of rhombohedral symmetry. They exhibit these superior properties only when they are actuated out of the spontaneous polarization direction, which is 111
when referred to the crystal axes of the corresponding cubic perovskite. The highest actuation to date appears to have been observed when the crystals are actuated with the applied field parallel to the 100 direction of the corresponding cubic perovskite.
A drawback to all these lead-based perovskites is the toxicity of lead and associated difficulties in processing, crystal growth and handling. In certain environmentally sensitive or biologically sensitive applications, they can not be used. Another drawback is the high density and relatively low elastic modulus of the lead based perovskites. For applications where toxicity or weight-based measures of performance are important, alternative materials are desired.
(Na1/2Bi1/2)TiO3 (NBT) and (K1/2Bi1/2)TiO3 (KBT) are known to crystallize in the perovskite crystal structure. See, Smolenskii et al., Soviet Phys. Solid State 2(11):2651 (May, 1961) and Roleder et al, Ferroelectrics 89:1 (1989). At room temperature, NBT is a rhombohedral ferroelectric with a Curie temperature of 320° C. Undoped BNT has been grown as single crystals, and the structure and phase transitions as a function of temperature have been studied; however, the piezoelectric properties of the single crystal are not known and it has not been used as a piezoelectric actuator.
(Na1/2Bi1/2)TiO3 has also been alloyed with alkaline earth titanates. Polycrystalline materials in the (Na1/2Bi1/2)TiO3-M′TiO3 (M′=Ca, Sr, Ba, Pb) systems have been previously studied. See, Takenaka et al. Ferroelectrics 7:347 (1974); and Jap. J. Appl. Phys. 30(9B):2236 (1991). In all these doped materials, the modest values of d33 and k33 make them of relatively limited interest for piezoelectric actuation, when compared with materials such as PZT.
Takenaka et al. reports the alloying of (Na1/2Bi1/2)TiO3 with BaTiO3 (Jap. J. Appl. Phys. 30(9B):2236 (1991)) or PbTiO3 (Elec. Eng. Japan 112(7):92 (September 1992)). The compounds exhibit a morphotropic phase boundary (MPB) between rhombohedral (ferroelectric) and tetragonal (ferroelectric) phases at approximately 6 molar % and 13.5 molar % of the additive, respectively, at 20° C. Near the morphotropic phase boundary, d33 values of approximately 125 pC/N and k33 of about 55% have been reported.
For (Na1/2Bi1/2)TiO3 alloyed with CaTiO3, the morphotropic phase boundary between rhombohedral (ferroelectric) and cubic (paraelectric) phases lies at approximately 17% at 20° C., and a 10% composition shows d33 values of approximately 50 pC/N and k33 of about 38%. See, Takenaka et al. Japan. J. Appl. Phys., 28:59 (1989).
(Na1/2Bi1/2)TiO3 alloyed with SrTiO3 shows a more complicated sequence of phases with increasing Sr concentration. A rhombohedral phase (antiferroelectric) phase intervenes between the rhombohedral (ferroelectric) and cubic (paraelectric) endmembers. Some mixed substitutions such as those between NBT and (Pb,Ba)TiO3 and (Pb2Sr)TiO3 have also been characterized and show, near the morphotropic phase boundary (MPB) between rhombohedral (ferroelectric) and tetragonal (ferroelectric) phases, d33 125=pC/N and k33=55%. See, Takenaka et al. Ferroelectrics 7:347 (1974).
While the intrinsic electromechanical properties of properly oriented single crystals appear to be attractive, there are numerous obstacles to their practical implementation. Firstly, the growth, orientation, cutting and handling of large single crystals is expensive and difficult. For example, a typical method of single crystal growth is the Czochralski method in which a crystal may take days to grow. Special precautions in crystal growth may have to be taken for compounds that contain toxic and volatile components, such as lead oxide. Secondly, ceramic ferroelectrics are mechanically brittle and weak, more so than other ceramics such as aluminum oxide, silicon nitride or silicon carbide. The act of electrical actuation can be sufficient to cause fracture in a single crystal actuator.
Polycrystalline materials are more tolerant of crystalline defects and are easier to manufacture. Nonetheless, a conventional polycrystalline ferroelectric, electrostrictive or magnetostrictive actuator suffers in performance relative to an optimally oriented single crystal. One reason is that randomly oriented crystallites can not simultaneously be oriented with respect to an applied field so as to optimize the intrinsically anisotropic crystal properties in each grain. Another reason is that each single crystal grain undergoes anisotropic crystal deformation under the applied field and so works against its neighbors. This limits the net strain which is achievable from polycrystalline actuator material.
If the majority of the gains in a polycrystal are textured, that is, share one or more common crystallographic directions, the electromechanical properties can approach those of a single crystal. However, it is well-known to those skilled in the art that polycrystalline ceramics are difficult to prepare in a textured form. Ceramics which tend to grow grains with an anisotropic platelet shape, such as micaceous minerals or the Bi—Sr—Ca—Cu—O superconducting compound, or a needle-like shape, such as certain magnetic oxides, can sometimes be prepared in an aligned form by deformation and/or recrystallization of an initial isotropic grain or by applying a high electric or magnetic field. Systems with isotropic grain and crystallite shapes, which include the perovskites of interest, are much harder to align. While oriented polycrystalline perovskite ferroelectrics have been accomplished in the form of thin films supported by a substrate they have not been achieved in polycrystals suitable for actuation devices.
It is an object of the present invention to provide perovskite compounds which are low in lead or lead-free and which can be grown as single crystals for use as electromechanically active materials.
It is a further object of the invention to provide electromechanically active perovskite compounds of rhombohedral, tetragonal and other phase symmetries for use in electromechanical actuators.
It is a further object of the invention to provide perovskite compounds which are low in lead or lead-free and which have superior electromechanical properties, such as improved strain coefficients, low hysteresis piezoelectric actuation, ultrahigh room temperature electrostrictive actuation, improved coupling constant and/or improved actuation strain.
Yet a further object of the invention is to provide a piezoelectric composite device which provides the advantages of anisotropic properties of large single crystal materials without the need to use said large crystals.